The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:
3GPP third generation partnership project
BS base station
CB coordinated beamforming
CoMP coordinated multipoint transmission/reception
CS coordinated scheduling
CSI channel state information
CSI-RS CSI-reference symbols
CQI channel quality indicator
DM-RS demodulation reference symbols
eNB evolved NodeB (BS of a LTE/LTE-A system)
JP joint processing
JT joint transmission
LTE long term evolution (evolved UTRAN)
LTE-A long term evolution advanced
MIMO multiple-input multiple-output
MU multi-user
PMI precoding matrix indicator
RF radiofrequency
RI rank indicator
RRH remote radio head
RSRP reference signal received power
SINR signal to interference-plus-noise ratio
SU single-user
TX transmission
UE user equipment
UTRAN universal terrestrial radio access network
In the 3GPP LTE and LTE-A systems, interference is a limiting factor for single cell single-user (SU-) and multi-user (MU-) multiple-input multiple-output (MIMO) network performance. This is particularly true at the cell edge, and for this reason CoMP has been introduced. In the CoMP concept, for the DL there are multiple points which cooperate in scheduling and transmission for a single UE in order to strengthen the desired signal and to mitigate inter-cell interference. In practice these multiple transmission points may be base stations (eNBs in the LTE/LTE-A systems), remote radio heads and the like, or some combination of them. In the 3GPP definition (see 3GPP TR 36.819) for CoMP, a set of geographically co-located transmit antennas and the sectors of the same site correspond to different points. It should be noted that a cell is formed by one or multiple points.
The above 3GPP technical report was approved after the RAN#1 meeting and it was agreed to focus further work for specifying CoMP in the LTE/LTE-A systems on joint transmission (JT), dynamic point selection (DPS, including dynamic point blanking/muting), and coordinated beamforming/coordinated scheduling (CB/CS, also including dynamic point blanking/muting).
In JT CoMP two or more points transmit simultaneously to a CoMP user. In DPS CoMP the transmission point is switched according to changes in signal quality. In CB/CS CoMP the scheduling decisions of neighbor points are coordinated in order to reduce interference. In principle any of these CoMP schemes may include blanking/muting which means that transmissions from one or more transmission points are blanked/muted in order to decrease their interference to one or more other CoMP transmission points which is not blanked/muted.
There are at least four distinct CoMP scenarios to consider, known as follows among the 3GPP study group/work item for CoMP. These address both intra-cell and inter-cell CoMP schemes operating in homogeneous and in heterogeneous network deployment environments. Scenario #1 is intra-site CoMP; scenario #2 is inter-site CoMP with a high power RRH; scenario #3 is CoMP with a low power RRH within the coverage area of the macro cell and having different cell identities IDs; and scenario 4 is the same as scenario 3 but with the same cell identities between the macro and the RRHs. CoMP for LTE/LTE-A is to be operative for both frequency division duplexing FDD and time division duplexing TDD, and so it is advantageous that solutions for how to implement CoMP are unified for both duplexing schemes. These teachings are relevant to CSI feedback from the UE to the eNB in support of at least these above CoMP techniques.
From the UE's perspective there are two different sets for CoMP. There is a CoMP measurement set, formed by M cells/points for which the UE is measuring channel state information CSI. And there is the UE's CoMP reporting set, which is the N cells/points defining the number of points involved in the actual CoMP scheme and so is made up of the points for which CSI feedback is reported. It is assumed that the CoMP reporting set will typically consist of two to three points. The CoMP reporting set could be equivalent to the CoMP measurement set so members of the reporting set N are elements of the measuring set M. The number of points involved in a CoMP scheme (those transmission points which are cooperating) does not need to be signaled to the UE or mentioned in specifications according to the current understanding in 3GPP but is left for network implementation. The point from which the UE would receive a transmission in single-cell mode is defined as the serving point.
In 3GPP Release 10 there are different reference signals defined for CSI estimation (CSI-RS) and data demodulation (DM-RS) purposes. There is also specified PDSCH resource element muting, which allows for multi-cell channel estimation. It is agreed that CSI feedback will be implicit, consisting of rank indicator (RI) report, precoding matrix index (PMI) and a channel quality indicator (CQI). From the UE's perspective it will estimate the channel, select the rank and PMI and calculate the post-processing (after receiver) SINR and from those derive the CQI. Thus the CQI in this approach may be seen as indicative of the post processing SINR. Release 10 feedback operates per point, but certain CoMP variations provide that a UE may receive CSI-RS resources from more than one point. Additionally, it is possible to have a single feedback that is aggregated over multiple CSI-RS resources, or per point (per CSI-RS resource). Further, the per-point PMIs may be improved by a combiner feedback, which is an inter-point phase and/or an amplitude value. FIG. 1 summarizes the feedback and channel estimation options for each of the above CoMP schemes.
Since there are several different transmission schemes/hypothesis outlined above, a problem arises in that the CQI feedback from the UE may be computed on a transmission scheme/hypothesis the UE assumes is in use which does not match what the network is actually employing. The network uses CQI for scheduling and to perform adaptive modulation and coding, which means the transmission rate is adapted based on channel conditions as reported by the UE. Inaccuracy in the CQI value can greatly affect system performance. For example, if CQI is reported to be better than the actual channel conditions the network may assign a transmission rate that is too high for the radio link to support, resulting in high error rates and re-transmissions which as a consequence degrade throughput.
The CQI which the UE reports in a CoMP environment depends on the transmission scheme/hypotheses made by the UE at a given time. For example:                When reporting an aggregated JT CQI, the UE assumes there was a simultaneous transmission from N points to the UE.        When reporting a DPS CQI without muting, the UE assumes the transmission was from a (single) selected point and there may be interference from other points.        When reporting a DPS CQI with muting, the UE assumes the transmission was from one point and that there is zero interference from the other points that are assumed to be muted.        
So there are multiple methods of calculating the CQI, and the CQI which the UE reports depends on the CoMP transmission scheme/hypothesis the UE assumes. What is needed in the art is a way for the UE and the network to have a common understanding of the CoMP scheme relevant to the CQI being reported by the UE.